There has been considerable interest in developing high resolution measurement devices for evaluating sample parameters. This need is particularly acute in the semiconductor industry where very thin films, such as oxides, metals or dielectrics, are deposited on semiconductor or metal substrates such as silicon. Non-destructive techniques are particularly needed to evaluate thickness, impurities and index of refraction characteristics of the films to insure high yields during fabrication.
One such inspection device which has been successful in this field is marketed by the assignee herein under the trademark Optiprobe. This device includes an optical measurement system described in U.S. Pat. No. 4,999,014, issued Mar. 12, 1991, to Gold. This system evaluates a sample based on interference effects created when a probe beam interacts with a thin film layer deposited on the surface of a substrate.
In the method described in the Gold patent, a probe beam is focused onto the surface of the sample in a manner to create a spread of angles of incidence. The intensity of rays within the reflected probe beam is measured as a function of the radial position of the ray within the beam using a diode array. The radial position of each ray can be directly related to the angle of incidence that ray with respect to the sample. A processor is used to evaluate the measured intensity of the rays, as a function of angle of incidence, to determine information about the sample.
The concept of obtaining multiple angle of incidence measurements described in U.S. Pat. No. 4,999,014, has more recently been extended to ellipsometry. An ellipsometric system is described in U.S. Pat. No. 5,042,951, issued Aug. 27, 1991, to Gold. The latter patent describes how prior art ellipsometric principles can be enhanced using a simultaneous multiple angle of incidence measurement technique.
In conventional ellipsometric techniques, a probe beam having a known polarization state is directed to interact with the sample. An analyzer is provided to determine the polarization state of the beam after it has interacted with the sample. The change in polarization state of the beam caused by its interaction with the sample is a function of the sample parameters and measurement of this change allows the sample to be analyzed. In a typical measurement scenario, the azimuthal angle of the polarizing or analyzing elements are varied to obtain multiple measurements.
U.S. Pat. No. 5,042,951 describes how ellipsometric information can be obtained at multiple angles of incidence simultaneously. This result is achieved by analyzing the change in polarization state of individual rays within the probe beam as a function of the radial position of the rays. As noted above, the radial position of the rays in the reflected probe beam can be related to the angle of incidence of the rays on the sample. Using an array detector, the ellipsometric parameters are determined simultaneously at multiple angles of incidence.
The measurement approaches described above require analysis of the output of individual elements of a photodetector array. In practice, analysis of these signals can prove challenging since the output of individual detector elements is relatively low.
To overcome this problem, an alternative approach was developed and is described in U.S. Pat. No. 5,181,080 issued Jan. 19, 1993 to Fanton. In the approach described in the latter patent, signal detection is enhanced by using a quad-cell photodetector rather than the individual detector elements of an array to measure changes in a probe beam. In operation, a linearly polarized probe beam is tightly focused onto the surface of the sample to create a spread of angles of incidence. The reflected probe beam is passed through a quarter-wave plate to retard the phase of one of the polarization states by ninety degrees. The beam is then passed through a linear polarizer aligned at 45 degrees with respect to the axes of the quarter-wave plate. The polarizer functions to create interference effects between the two polarization states of the beam.
The interference between the two polarization states produces a signal which is proportional to the characteristics (such as thickness) of the film on the sample. This signal, which is a component of the total power of the reflected beam, represents an integration of a plurality of rays at multiple angles of incidence. The relevant signal has a positive sign in two opposed quadrants of the beam and a negative sign in the remaining two quadrants of the beam. Therefore, the relevant signal can be isolated if the entire beam is focused onto the surface of a photodetector which has four detection regions laid out in quadrants. The output signals from two opposed quadrants can be summed and subtracted from the sum of the output signals from the remaining two quadrants. The result of this calculation produces a value which is proportional to certain characteristics of the film, such as film thickness. The approach described in this patent has been implemented in an upgraded version of the device referred to above and is marketed under the designation Optiprobe 2000.
Using a combination of interference measurements and integrated ellipsometry, highly accurate information can be obtained for a variety of films on a variety of substrates. It is well known, however, that the analysis of a sample can be further enhanced if optical measurements are taken at more than one wavelength of light. For example, certain films will exhibit a greater signal response to certain wavelengths. More importantly, obtaining measurements at multiple wavelengths for a given sample can help reduce ambiguities in the analysis. This approach can be particularly useful in situations where the index of refraction of a film is not accurately known or the evaluation is being performed on samples with multiple film layers.
Measurements at multiple wavelengths can be performed relatively easily in a sequential manner. For example, the probe beam can be generated by a white light source. A filtering mechanism is then placed in the path of the white light source and functions to selectively transmit various wavelengths. The filtering mechanism could be defined by a grating, prism or color wheel. Various sequential measurements can then be taken at different wavelengths. The main drawback to this approach is that multiple sequential measurements at different wavelengths can be quite time consuming.
In order to increase the speed of operation, it would be desirable to obtain measurements at multiple wavelengths simultaneously. To achieve this goal, an unfiltered polychromatic light beam is focused onto the sample. After the probe beam has interacted with the sample, some form of dispersing element (prism or grating) can be used to split the probe beam into various wavelength components. This approach is common to many prior art spectrophotometers.
Unfortunately, complications arise when measurements are to be made both at multiple wavelengths and multiple angles of incidence simultaneously. These complications were addressed in copending application Ser. No. 08/093,178, assigned to the same assignee as herein. This application describes an approach wherein a filter having a rectangular slit is used to transmit a narrow band of light lying along a diameter of the probe beam. The filter is positioned in an image plane of the focusing lens to achieve the desired effect. The polychromatic beam passing through this filter is then angularly dispersed as a function of wavelength. Using one or more two dimensional arrays of photodetectors, multiple angle of incidence information at multiple wavelengths could be obtained simultaneously.
The latter application also discloses an embodiment for measuring an ellipsometric signal which is an integration of the multiple angles of incidence. In this embodiment, a filter having a narrow rectangular slit is oriented at 45 degrees with respect to the initial polarization state of the beam. A quarter-wave plate and a polarizing beam splitter are provided to divide the beam into left-hand and right-hand circular polarizations. A pair of dispersing elements are provided to angularly separate the pair of beams as a function of wavelength in a manner to strike a pair of two dimensional array detectors. The output of one row of detector elements of one array is subtracted from the corresponding row of detector elements on the other array to obtain the integration signal of interest at a particular wavelength. Different rows in each array correspond to different wavelengths. It is also possible to subtract the output of a given pixel in one array from the output of the corresponding pixel in the other array to obtain specific multiple angle of incidence information at multiple wavelengths.
The latter approach for obtaining an integrated ellipsometric signal at multiple wavelengths is feasible to implement. However, it is believed that the alternative approach described herein will provide improved results.